1. Field of the Invention
The present invention relates to a wireless mobile communication system, and more particularly to a communication system based on a Multiple Input Multiple Output (MIMO) scheme.
2. Discussion of the Related Art
MIMO technology is an abbreviation for Multiple Input Multiple Output technology. MIMO technology uses a plurality of transmission (Tx) antennas and a plurality of reception (Rx) antennas to improve the efficiency of transmission and reception (Tx/Rx) of data. In other words, MIMO technology allows a transmission end or reception end of a wireless communication system to use multiple antennas (hereinafter referred to as a multi-antenna), so that the capacity or performance can be improved. For convenience of description, the term “MIMO” can also be considered to be a multi-antenna technology.
In more detail, MIMO technology is not dependent on a single antenna path to receive a message. Instead, the MIMO technology collects a plurality of data fragments received via several antennas, merges the collected data fragments, and completes total data. As a result, MIMO technology can increase a data transmission rate within a predetermined-sized cell region, or can increase system coverage while guaranteeing a specific data transmission rate. Under this situation, MIMO technology can be widely applied to mobile communication terminals, repeaters, or the like. MIMO technology can extend the range of data communication, so that it can overcome the limited amount of transmission data of mobile communication systems.
FIG. 1 is a block diagram illustrating a general MIMO communication system.
Referring to FIG. 1, the number of transmission (Tx) antennas in a transmitter is NT, and the number of reception (Rx) antennas in a receiver is NR. In this way, theoretical channel transmission capacity of the MIMO communication system when both the transmitter and the receiver use a plurality of antennas is greater than that of another case in which only the transmitter or the receiver uses several antennas. The theoretical channel transmission capacity of the MIMO communication system increases in proportion to the number of antennas. Therefore, data transmission rate and frequency efficiency are greatly increased. Provided that a maximum data transmission rate acquired when a single antenna is used is set to Ro, a data transmission rate acquired when multiple antennas are used can theoretically increase by a predetermined amount that corresponds to the single antenna data transmission rate (Ro) multiplied by a rate of increase Ri. The rate of increase (Ri) can be represented by the following equation 1.Ri=min(NT,NR)  [Equation 1]
For example, provided that a MIMO system uses four transmission (Tx) antennas and four reception (Rx) antennas, the MIMO system can theoretically acquire a high data transmission rate which is four times higher than that of a single antenna system. After the above-mentioned theoretical capacity increase of the MIMO system was demonstrated in the mid-1990s, many developers began to conduct intensive research into a variety of technologies which can substantially increase a data transmission rate using the theoretical capacity increase. Some of the above technologies have been reflected in a variety of wireless communication standards, for example, a third-generation mobile communication or a next-generation wireless LAN, etc. The above-mentioned MIMO technology can be classified into a spatial diversity scheme (also called a Transmit Diversity scheme) and a spatial multiplexing scheme. The spatial diversity scheme increases transmission reliability using symbols passing various channel paths. The spatial multiplexing scheme simultaneously transmits a plurality of data symbols via a plurality of transmission (Tx) antennas, so that it increases a transmission rate of data. In addition, the combination of the spatial diversity scheme and the spatial multiplexing scheme has also been recently developed to properly acquire unique advantages of the two schemes.
In association with the MIMO technology, a variety of MIMO-associated technologies have been intensively researched by many companies or developers, for example, research into an information theory associated with a MIMO communication capacity calculation under various channel environments or multiple access environments, research into radio frequency (RF) channel measurement and modeling of the MIMO system, and research into a space-time signal processing technology for increasing transmission reliability and data transmission rate.
In a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) system, the above-mentioned MIMO scheme is applied to only downlink signal transmission of the 3GPP LTE system. The MIMO technology may also be extended to uplink signal transmission. In this case, a transmitter structure should be changed to implement the MIMO technology, so that a Peak power to Average Power Ratio (PAPR) or Cubic Metric (CM) characteristics may be deteriorated. Therefore, there is needed a new technology capable of effectively applying the MIMO scheme to uplink signal transmission.
Specifically, an appropriate number of precoding matrices are selected from a codebook for use in uplink rank 3 transmission, and at the same time it is necessary for the codebook to maximize a chordal distance between precoding matrices.